Axial fan impellers having an impeller body with a number of outwardly protruding blades and a hub which carries the blades and can be fastened on the rotor of an external-rotor motor are generally known.
Also known are axial fan impellers of the type described above in which the hub is formed in a tubular manner and can be pressed onto the rotor for fastening. These fans further include a plastic-encapsulated, substantially cylindrical, metallic blank being arranged inside the hub and circumferentially enclosing the rotor of the external-rotor motor. The cylindrically shaped steel blank of the known plastic impellers is expanded during the pressing-on operation. This expansion may cause high mechanical stresses in the hub, which can cause the impeller to break. Further disadvantages are that it is not possible for condensation that may form in the hub in cases where the temperature drops below the dew point to run off and that the plastic hub hinders the heat dissipation via the surface of the rotor.
The invention is based on the object of providing an impeller of the type previously described with greater mechanical stability in such a way that it can be produced with little technical complexity. In particular an impeller of this invention allows mechanical stresses in the hub arising as a result of the operation of pressing it onto a rotor shaft to be reduced.
This object is achieved according to the invention by the blank having alternately arranged first and second portions, the first portion having a greater inside diameter than the inside diameter of the second portion.
Consequently, a segmented metallic blank, in particular a steel blank, is preferably used according to the invention, “segmented” being understood as meaning that the cylindrical part of the blank is subdivided into portions with different inside diameters. The portion with the smaller diameter is widened during the pressing-on operation, while the portions with the large segment diameter do not change their radial position, or only slightly, during the pressing-on operation, but advantageously bring about a lowering of the average level of the mechanical stress.
While in the region of the greater diameter, where only a slight deformation occurs during assembly, the hub may, in a preferred configuration of the invention, be joined to the blank by means of webs, for the purpose of providing high mechanical stability. A plastic encapsulation of the blank may be formed in such a way that pockets are incorporated in the hub preferably in the region of the small diameter. In this way, the deformation of the blank occurring during pressing-on is not imposed on to the hub.
The pockets in the hub may in this case also be advantageously formed continuously in the axial direction, so that a connection is established between the suction side and the pressure side of the fan. As a result, two further advantages are also achieved. Specifically, a runoff of condensation possibly forming is made possible and an air flow occurs between the suction side and the pressure side through the pockets improving the cooling of the rotor.